1. FIELD OF THE INVENTION
The present invention relates to a method for a dynamic or real time control of a steel making process involving decarburization of molten steel in a closed zone under reduced pressures and compulsive evacuation of a exhaust gas comprising CO, CO.sub.2 and N.sub.2 from the closed zone. Particularly, the present invention relates to such a method wherein the carbon content of the steel at the end point of the decarburization process may be precisely controlled to a preset value by promptly detecting the carbon content or rate of decarburization of the molten steel being processed at any desired instance and by controlling the process in accordance with the detected carbon content or rate of decarburization.
2. BRIEF DESCRIPTION OF PRIOR ART
Widely practiced is a steel making process which involves decarburization of molten steel under reduced pressures. In one typical process for producing stainless steel which is generally referred to as the VOD process, chromium-containing molten steel is vacuum decarburized in a ladle mounted in a closed vessel by blowing oxygen onto the molten steel which may be stirred by bubbling argon.
Recent progress in the art has made it possible to produce various kinds of steel, and in consequence, it has become increasingly important to promptly detect and determine certain parameters indicative of the state of the molten steel being processed and to control the process in accordance with the determined values of the parameters so that the desired steel may be produced. Among others detection of the carbon content of the molten steel being processed is particularly important, because the primary object of the process is to decarburize the molten steel. However, it is not easy to precisely and instantaneously detect the changing carbon content of the molten steel every moment, which steel is being processed under vacuum in a closed vessel.
For the determination of the carbon content of the molten steel and control of the end point thereof various methods have heretofore been proposed, including, for example, an experimental method based on static analyses; a method wherein a rate of decarburization of the molten steel is indirectly predicted from a change in the monitored degree of vacuum of the exhaust gas, and; a method wherein a partial pressure of oxygen in the exhaust gas is monitored by means of a concentration cell, and from an inflection point of the change in the monitored partial pressure of oxygen the carbon content of the molten steel is determined. However, such known methods are unsatisfactory because of poor precision and, it has been difficult to successfully control various kinds of steel containing various quantities of Cr, Ni and Mn to end carbon levels differently desired to the respective particular products.
One approach to the problem is to precisely and instantaneously measure the amount of carbon which has been transferred to the exhaust gas, that is the quantities of CO and CO.sub.2 inthe evacuated exhaust gas. Attempts have been made to measure the quantity of the exhaust gas caused to flow through a duct communicating the vacuum vessel and evacuation means, as well as the quantities of CO, CO.sub.2 and O.sub.2 in the exhaust gas. Infrared gas analyzers for analyses of CO and CO.sub.2 and a magnetic gas analyzer for analysis of O.sub.2 have heretofore been utilized. However, such instruments have a limited precision and response speed so that it is difficult to know a precise carbon level of the molten steel every moment from information obtained with such instruments. Moreover, these instruments have been inherently designed for analyses of gases under atmospheric pressure, and therefore, upon analysis of a gas under a reduced pressure a waiting period is required before a sufficient volume of the gas for analysis accumulates, rendering the response speed and precision of the instruments still worse. In order to avoid such a waiting period, a proposal has been made wherein the evacuated exhaust gas is sampled for analysis purpose at the discharge side of the evacuation means. However, no satisfactory results have been obtained by such a proposal. This is because the measurement involves errors corresponding to the fraction of the CO.sub.2 which is dissolved in water condensed by the condensers 6a through 6d and removed from the system. Furthermore, the fact that different gas analyzers are required for detecting different gaseous components in a sample of the exhaust gas poses difficulties in handling errors and time-lags of the respective analyzers.
In Japanese Patent Laid-open Specification No. 50(1975)-99592, published on Aug. 7, 1975 and assigned to the assignee of the present application, we have disclosed a method of determining the quantity of a gas formed in a gas producing chamber, such as the quantity of steam formed in a drier. The method proposed therein comprises the steps of feeding a dummy gas to the gas producing chamber, monitoring the quantity of the dummy gas fed to the gas producing chamber as well as the partial pressures of the dummy gas and the gas formed contained in an exhaust gas, and determining the quantity of the gas formed from the monitored values. The laid-open specification further teaches that the partial pressures of the gases may be advantageously measured by a mass spectrometer, and suggests that the proposed method may be applicable for determination of gases formed in a steel making furnace. However, this laid-open specification is completely silent with respect to difficulties inherently involved in mass spectrometrical analysis of a gas comprising CO, CO.sub.2 and N.sub.2. In fact, parent peaks for CO and N.sub.2 in a mass spectrum are inseparable because CO and N.sub.2 have the same mass number of 28. Moreover, a fragment peak for CO.sub. 2 appears at a mass number of 28 and perturbs the parent peak for CO which also appears at the same mass number. Furthermore, there are additional difficulties in the mass spectrometry of the exhaust gas compulsively evacuated in the decarburization process of molten steel under reduced pressures, owing to the fact that on one hand a sensitivity of a mass spectrometer for a gas varies depending upon the pressure of the gas, and on the other hand the pressure of the evacuated exhaust gas varies to a great extent in the course of vacuum decarburization of the molten steel. The laid-open specification is also completely silent with respect to such difficulties.